This invention relates to cylinder bed sewing machines and more particularly to a thread trimming mechanism therefor.
Thread cutters for sewing machines are well known in the art. It is also well known in the art to provide thread cutters wherein the trimming mechanism is movable between the top side of the looper and the underside of the throat plate. An example of these type thread cutters are shown in U.S. Pat. Nos. 3,424,115 and 3,424,117 to D. Schopf and granted Jan. 28, 1969. It is also known in the art to equip a cylinder bed machine with a thread cutter. An example of the foregoing being set forth in a machine manufactured by Union Special G.m.b.H. and sold as a style 33700 KE or KG machine.
In view of the space constraints within the cylinder bed of the present machine it has been found that a linear looper drive has advantages over a pivotally mounted looper. Some of the advantages of employing a linear drive as compared with a pivotally mounted looper being that the drive for same may be remote from the looper itself, and the linear drive is a direct drive and does not require a plurality of linkages to deliver the driving force to the looper. The above identified patents, as well as most of the sewing machine industry, has heretofore employed pivotally driven looper assemblies. However, when a looper mechanism is changed from pivotal motion to linear motion as disclosed in the present invention, certain demanding constraints are placed on other mechanisms of the machine.
As particularly applicable to the present invention, the changeover from a pivotally movable looper to a linear moving looper greatly effects the path of movement for the thread trimming mechanism. That is, the path of movement of the thread mechanism, which in turn influences the effectiveness of the trimming mechanism, must be such that the thread loop depending from the workpiece and situated about the looper must be positively severed in order to produce an acceptable garment. In this regard, one of the constraints in a trimming mechanism for chainstitch machines is that said mechanism must approach the depending thread loop with a path of travel generally parallel with the motion of the looper in order to assure that the loop is affected. This parallelism approach is even more important when the invention is employed with a multiple needle machine wherein each of the depending loops has to be affected to insure an acceptable garment. As may be appreciated, if it were proposed in a multiple needle machine to angularly offset the approach of the trimming mechanism to the thread loops the chance of missing any number of the loops is greatly enhanced due to the inclination of approach with respect to the loops of thread.
The thread cutters shown in the two above identified patents do not lend themselves to the present invention for the following reasons. Both of the patented devices require that the pivot point of the cutting assembly be sufficiently displaced from the stitch forming area so as to impart to the cutter blades an arcuate path having a sufficient linear approximation so as to assure that the blade passes through the center of the thread loops formed about the looper as has been heretofore discussed. When employing multiple needle machines, as may be the case with the present invention, the problem is enhanced because the thread loops are spaced along the longitudinal axis of the looper and thus the distance between the pivot point and the arcuate path travelled by the cutter blade has to be extended to insure that the blade will have a sufficient linear approximation so as to enter all of the thread loops. As mentioned above, the present invention is embodied in a cylinder bed machine wherein the base casting or cylinder bed does not lend itself to sufficient room enabling displacement of a pivot point a removed distance from the stitching area through which the cutting blade must travel. Therefore, the thread cutters shown in these patents do not readily lend themselves to incorporation within the machine of the type hereunder consideration.
Turning now to the embodiment of a straight line thread cutter as employed in the above mentioned class 33700 machines, it may be seen why a thread cutter of this sort is most difficult to incorporate within the cylinder bed machine of the present invention. It should be pointed out, that a Class 33700 KE or KG machine is provided with a pivotally moving looper assembly. The trimming mechanism on said machine is placed in a parallel aligned relationship with the path travelled by the looper. The trimming mechanism is capable of being situated in this position because the arcuate path of the looper, and more particularly the arcuate path of the heel of the looper, passes under the removed or non-cutting position of the trimming mechanism and thus allows close access of the mechanism to the stitch forming instrumentalities without any interference resulting. However, in the present invention, there is employed a linearly moving looper. In this respect if the trimming mechanism embodied in the present invention were situated in a parallel aligned relationship with the path travelled by the looper, as is disclosed in the above mentioned machine, an interference between the path travelled by the heel of the looper and non-cutting position of the trimming mechanism would occur. Therefore, the trimming mechanism as is disclosed in a Class 33700 machine does not lend itself to the present invention.
In summation, the constraints inherent with a cylinder bed machine employing a linear moving looper requires that the trimming mechanism approaches the needle thread loops in a path generally parallel and aligned with the looper path while at the same time the non-cutting position of the trimming mechanism must lie sufficiently away from the linear path of the heel of the looper so as to avoid any interferences between the looper mechanism and the trimming mechanism. For the reasons discussed hereinabove, the art heretofore known does not readily lend itself to such an application.